[paulmd]

You can reach herd immunity on any one strain - but as we've seen, the virus can mutate faster than we can reach herd immunity.

The arrival of delta (and the other four or five major strains) signal that that theory flat-out will not work, the virus is mutating faster than we are reaching herd immunity. By the time we actually reach full herd immunity, we will have strains that the vaccine only provides significantly diminished protection against - we are arguably already getting there with Delta.